BACKGROUND OF THE INVENTION
a) The Field of The Invention
[0001] The present invention relates to a process for the preparation of Decabromodiphenyl
ether. More particularly, the present invention relates to a process which employs
a mixture of halogenated organic solvents, by which Decabromodiphenyl ether is obtained
having improved thermal stability.
[0002] Decabromodiphenyl ether, hereinafter referred to as "DECA" for the sake of brevity,
is a well known flame retardant agent, useful in the preparation of articles made
of polymeric material, to which it is desired to impart flame-retardant properties.
b) The Prior Art
[0003] According to the known art DECA is prepared in a variety of solvents, ranging from
liquid bromine to halogenated organic solvents. U.S. Patent 4,521,633 discloses one
of such processes in which DECA is prepared by reacting diphenyl ether in methylene
chloride (dichloromethane) with a brominating agent in the presence of a catalyst,
by initiating the reaction at a temperature of 15°C or lower, and then raising the
temperature of the reaction mixture to an elevated temperature.
[0004] U.S. Patent 3,959,387 discloses a process for the preparation of polybrominated biphenyl
oxides in which the reaction is carried out in methylene bromide as the solvent, at
temperatures of from room temperature to 200°C.
[0005] The above and other processes according to the art employ substantially pure halogenated
solvents, such as substantially pure methylene chloride or methylene bromide. When
organic solvents are employed for the preparation of DECA, it is a generally accepted
principle in the art that the solvent must be a substantially pure solvent. Thus,
for instance, U.S. Patent 4,521,633 states that the use of methylene chloride is particularly
advantageous in that it exhibits very low susceptibility to transhalogenation. Similarly,
U.S. Patent 3,959,387 teaches that the use of methylene bromide as the solvent is
necessary to the conduction of the reaction.
[0006] The use of pure chlorine-containing solvents, according to the art, has the considerable
drawback of requiring costly and time-taking purification steps, because of the transhalogenation
that takes place during the reaction with solvents such as methylene chloride. Reaction
of diphenyl oxide with a brominating agent in methylene chloride in the presence of
a bromination catalyst inevitably causes some transhalogenation of the solvent to
take place, which results in the presence of measurable amounts of both bromochloromethane
and dibromomethane in the reaction mixture. These transhalogenation products must
be separated before the methylene chloride is reused in a subsequent run.
SUMMARY OF THE INVENTION
[0007] It has now been surprisingly found, and this is an object of the present invention,
that is is possible to prepare DECA in a mixture of halogenated organic solvents,
and to obtain a product with high yield and good quality.
[0008] It has further been found, and this is another object of the invention, that it is
possible to operate in a quasi-steady state manner, when producing DECA in several
subsequent batches according to the process of the invention.
[0009] It has also been found, and this is still another object of the present invention,
that, in order to obtain a product having good thermal stability, the process of the
invention must be carried out at a maximal reaction temperature that does not exceed
a predetermined limit.
DETAILED DESCRIPTION OF THE INVENTION
[0010] The process for the preparation of decabromodiphenyl ether according to the invention
is characterized in that diphenyl ether, or one or more partially brominated diphenyl
ether deriv ative(s), is brominated in a mixture of solvents comprising at least
two of the solvents dichloromethane, bromochloromethane and dibromomethane in the
presence of a bromination catalyst, the maximal reaction temperature not exceeding
80°C.
[0011] A preferred embodiment of the invention is characterized by the steps of:
a) preparing a solution of a brominating agent and a bromination catalyst in a mixture
of solvents comprising at least two of the solvents dichloromethane, bromochloromethane
and dibromomethane;
b) providing liquid dipheny ether or a partially brominated diphenyl ether derivative,
or a mixture of two or more such derivatives, in molten form, or in a solvent selected
from among dichloromethane, bromochloromethane and dibromomethane, or a mixture of
two or more of said solvents;
c) adding the said liquid diphenyl ether of brominated diphenyl ether derivative(s)
to solution (a) thereby initiating the reaction;
d) continuing the reaction at a temperature equal to or lower than 80°C;
e) recovering the product from the reaction mixture;
f) recovering the mixture of organic solvents from the reaction mixture; and
g) recycling the mixture of organic solvents to the reaction vessel, optionally adding
a solvent selected from among dichloromethane, bromochloromethane and dibromemethane
or a mixture thereof as a makeup.
[0012] It is of course possible to employ a solid starting material rather than a liquid
material. This while permissible is impractical, as it will be apparent to the man
of the art. According to a preferred embodiment of the invention, the partially brominated
diphenyl ether derivative is selected from Pentabromodiphenyl ether (PENTA) and Octabromodiphenyl
ether (OCTA). It should be noted that such partially brominated derivatives comprise
a mixture of differently brominated diphenyl ethers. Thus PENTA, for instance, characterizes
a product having asn average content of five bromine atoms per molecule. Likewise,
OCTA is not a single compound but a mixture of brominated derivatives having an average
content of eight bromine atoms per diphenyl ether moecule.
[0013] Preferably, step (c) is carried out at a temperature lower than 25°C, more preferably
about -5°C to 5°C.
[0014] According to a preferred embodiment of the invention the brominating agent is bromine.
According to another preferred embodiment of the invention, the bromination catalyst
is an aluminum catalyst, selected from among metallic aluminum, AlCl₃ and AlBr₃.
[0015] According to a still preferred embodiment of the invention the makeup consists essentially
of methylene chloride. According to a still more preferred embodiment of the invention
the process is carried out in a quasi-steady state manner, as herein defined. According
to this preferred embodiment, the total amount of make-up solvent mixture added to
the recycled solvent mixture and the proportions between dichloromethane, bromochloromethane
and dibromomethane in the said make-up are such that addition of the make-up to the
recycled solvent mixture will provide a solvent mixture having a content of dichloromethane,
bromochloromethane and dibromomethane substantially equal to that of the previous
batch. If diphenyl ether is added in solution, rasther than in its pure melted form,
then the solvent employed for preparing this solution will be considered as a part
of the makeup, for the above purposes.
[0016] As it will be readily appreciated by a person skilled in the art, the invention provides
a very economical and convenient process. After a batch has been prepared, the organic
layer is separated from the aqueous layer, which is formed from water added during
the separation steps, and the resulting solvent mixture is distilled without fractionation.
H₂O is removed by azeotropic distillation or by addition of a drying agent before
distillation. Of course, some organic solvent may
be lost during these operations, and a makeup may be required. Such a makeup can consist
of any of the solvents present in the mixture, or of a mixture thereof in any proportion,
if available from another source. Moreover, the makeup can be added so to obtain a
solvent mixture which is desirable for a certain reaction, e.g., a solvent mixture
having a specified reflux temperature, lower than 80°C.
[0017] Ideally, it is possible to start from any given solvent mixture and to recycle and
reuse it until nearly all the solvent has been converted to dibromomethane. For practical
purposes, however, it is preferred to operate with mixtures that do not contain too
high fractions of dibromomethane and, therefore, the makeup solvent will usually contain
amounts of dibromomethane as low as possible. Usually, it will be preferred to use
dichloromethane as the makeup solvent, for cost reasons.
[0018] According to the process of the invention it is possible to operate in a quasi-steady
state manner, when producing DECA in the solvent mixture of the invention. This is
achieved by adding to the solvent mixture recovered from the previous batch an amount
of solvent, having any predetermined desired proportions in the mixture, so that when
the make-up solvent has been added the resulting solvent mixture will have substantially
the same proportions between the three different solvents that were present at the
beginning of the previous batch, from which the body of the solvent was recovered.
Obviously, this presents the considerable advantage of affording a process that operates
substantially at the same conditions in each separate batch, and therefore gives the
same results as any other batch of DECA produced in this semi-steady state process.
[0019] As it will be apparent to a person skilled in the art, this process solves many problems
related to quality and operating parameters, since substantially identical reaction
conditions can be achieved each time. In practice, when operating in a quasi-steady
state, as herein defined, there is no need to carry out any treatment of the solvent
mixture, other than the inevitable removal of water, since when the same amount of
the same make-up solvent is added in each new batch to the solvent recovered from
the previous batch, the solvent mixture so obtained will be substantially the same
that was employed in the previous batch, having the same composition and behaviour
as in any one of the previous batches prepared during the quasi-steady state production.
[0020] Furthermore, it is possible, whenever required, to effect a bleeding of the recycled
solvent mixture, in order to reduce the level of any impurities which might accumulate
in the solvent mixture. The make-up added will then be able to replace also the solvent
removed through bleeding, and not only any solvent lost during operations in the previous
batch. In any case, quasi-steady state conditions can be obtained, with or without
bleeding, as it will be apparent to the man of the art. Usually, when the solvent
from a completed batch is evaporated and transferred to the next batch, some solvent
is left on purpose in the reaction vessel to simplify removal of solid impurities
that are soluble therein and which would otherwise deposit on the reactor walls and
be more dififcult to remove. Therefore an operation comparable to what is normally
termed "bleeding" is usually effected, although it is likewise possible to evaporate
all the solvent and to remove the solid from the reactor by any other means.
[0021] The thermal stability of a flame retardant agent such as DECA is a very important
requirement. In order to be usefully employed as additives to various plastics, flame
retardant agents must be colorless, since coloration of the flame retardant additive
results in the change in color of the article which incorporates it. Thermal instability
of the FR agent results in its change from colorless to coloured when heated, for
example, during the incorporation process. This change in color may derive from the
inherent thermal instability of the FR agent or from the presence of impurities in
the product, which impart a thermal instability to it. It is therefore clear that
thermal stability is of paramount importance for obtaining articles with acceptable
color after the incorporation process. DECA itself is thermally stable and derives
its thermal instability from impurities formed during its preparation, the exact nature
of which is unknown. The applicant has now surprisingly found that thermal stability
of the DECA produced according to the process of the invention is obtained, if the
reaction temperature does not exceed 80°C. When operating with the process of the
invention, using mixtures containing less than about 50% (v/v) dibromomethane, the
reflux temperature is lower than 75°C. Therefore, when operating with such mixtures
the condition for obtaining thermally stable DECA is always met, and the process can
be carried out without any special precautions regarding the reaction temperature.
[0022] The above and other characteristics and advantages of the invention will be better
understood through the following illustrative and non-limitative examples.
Example 1
[0023] To a one-liter flask equipped with a mechanical stirrer, a dropping funnel, a thermometer
and a reflux condenser there were added 430 ml of a mixture containing 26% v/v dichloromethane,
8% v/v bromochloromethane and 66% v/v dibromomethane, 7.5g of anhydrous AlCl₃ and
880g of bromine (5.5 moles). The contents of the flask were cooled to about 0°C and
a solution of 85 g (o.5 mole) of diphenyl ether in 20 ml dichloromethane was added
dropwise to the stirred mixture during 2 hours, while maintaining the temperature
between 0° and 5°C. After completion of the addition, the contents of the flask were
heated between 64° and 75°C during 5 hours. After this time, the reaction was stopped
by adding 60 ml of water, and unreacted bromine was bleached by adding a concentrated
aqueous sodium bisulfite solution. The aqueous layer was separated and the organic
phase was washed twice with 150 ml portions of water and was then neutralized with
an aqueous sodium hydroxide solution. The mixture was filtered and the white product
was washed with water and dried in a vacuum oven at about 70°C. HPLC analysis gave
a 96% content of DECA in the product (m.p. 302-304).
[0024] Thermal stability was checked by heating the product in an oven for 2 hours at 280°C.
Color deviation was checked by visual observation with reference to the unheated product.
After heating was completed no appreciable change of color was observed.
[0025] The organic layer from the filtrate was distilled and was found to contain 4% dichloromethane,
33% bromochloromethane and 63% dibromomethane. This mixture was reused in the subsequent
run, after addition of 125 ml dichloromethane as the makeup.
Example 2 (Comparative)
[0026] Example 1 was repeated, with the exception that 100% dibromomethane was used and
when the addition of the diphenyl ether was completed the contents of the flask were
heated at temperatures between 80°C and reflux (about 92°C) for 2-5 hours, until completion
of the reaction. The product was analysed by HPLC and was found to contain 96% DECA
(m.p. 302-304°C). The resulting product was heated at 280°C for 2 hours, after which
period it became colored.
Examples 3 through 8
[0027] Example 1 was repeated in different solvent mixtures. The results and main conditions
for each run are summarized in Table I below.
Example 9
Quasi-steady state operation
[0028] 190 ml of a solvent mixture containing 62% v/v dichloromethane, 29% v/v bromochloromethane
and 9% vv dibromomethane was mixed with 180 ml dichloromethane. Bromination of diphenyl
ether was carried out follo wing the procedure of Example 1, with
0.5 moles diphenyl ether, 5.5 moles bromine, 15 g AlCl₃, a maximal reaction temperature
of 52°C and a reaction time (post addition) of 7.5 hours. The reaction was repeated
several times, by recycling the solvent mixture as hereinbefore described. The main
reaction yield was 96%.
[0029] Table II below shows the composition of the solvent mixture at different stages of
the process, for three consecutive runs. It should be noted that in this laboratory
experiment no special care was taken to avoid solvent losses. In industrial operation,
however, solvent losses can be very much reduced by taking appropriate care during
operation, as apparent to the man of the art.
[0030] As it can be seen from the data in the above table, addition of the appropriate amount
of make-up results in a semi-steady state operation. The process may be continued
indefinitely, by adding the same amount of make-up, and the solvent mixture composition
will vary within relatively narrow limits from one batch to the other. Furthermore,
the make-up can be finely controlled, if desired, by means known to a person skilled
in the art, to obtain a narrow limit for the desired steady state.
[0031] The above examples have been provided for the purpose of illustration, and are not
intended to be limitative. Many variations can be effected in the various means and
procedures. For instance, different proportions can be employed in the solvent mixture,
or lower maximal reaction temperatures can be used, all without exceeding the scope
of the invention.
1. A process for the preparation of decabromodiphenyl ether, characterized in that
diphenyl ether or one or more partially brominated diphenyl ether derivative(s) is
brominated in a mixture of solvents comprising at least two of the solvents dichloromethane,
bromochloromethane and dibromomethane in the presence of a bromination catalyst, the
maximal reaction temperature not exceeding 80°C.
2. A process according to claim 1, characterized by the steps of:
a) preparing a solution of a brominating agent and a bromination catalyst in a mixture
of solvents comprising dichloromethane, bromochloromethane and dibromomethane;
b) providing liquid diphenyl ether or a partially brominated diphenyl ether derivative,
or a mixture of two or more derivatives, in molten form, or in a solvent selected
from among dichloromethane, bromochloromethane and dibromomethane, or a mixture of
two or more of said solvents;
c) adding the said liquid diphenyl ether or brominated diphenyl ether derivative(s)
to solution (a), thereby initiating the reaction;
d) continuing the reaction at a temperature lower than about 80°C;
e) recovering the product from the reaction mixture;
f) recovering the mixture of organic solvents from the reaction mixture; and
g) recycling the mixture of organic solvents to the reaction vessel, optionally adding
a solvent selected from among dichloromethane, bromochloromethane and dibromomethane
or a mixture thereof as a make-up.
3. A process accoding to claim 1 or 2, wherein the partially brominated diphenyl ether
derivative is selected from Pentabromodiphenyl ether and Octabromodiphenyl ether.
4. A process according to claim 2, wherein step (c) is carried out at a temperature
lower than 25°C, preferably about -5°C to 5°C.
5. A process according to any one of claims 1 to 4 characterized in that the brominating
agent is bromine.
6. A process according to any one of claims 1 to 5, characterized in that the bromination
catalyst is an aluminum catalyst selected from among metallic aluminum, AlCl₃ and
AlBr₃.
7. A process according to any one of claims 2 to 6, characterized in that the make-up
consists essentially of methylene
chloride.
8. A process according to any one of claims 2 through 7, characterized in that it
is carried out in a quasi-steady state manner.
9. A process according to claim 8, wherein the total amount of make-up solvent mixture
added to the recycled solvent mixture and the proportions between dichloromethane,
bromochloromethane and dibromomethane in the said make-up are such that addition of
the make-up to the recycled solvent mixture will provide a reaction mixture having
a content of dichloromethane, bromochloromethane and dibromomethane substantially
equal to that of the previous batch.